1. Field of the Invention
The present invention relates to an optical scanning device and an optical scanning method.
2. Description of the Related Art
In a high-resolution image forming apparatus, it is desired that an image writing position by means of optical scanning is controlled to a predetermined position within an effective image forming area in which an image is formed with respect to a main-scanning direction and a sub-scanning direction.
Generally, as to the main-scanning direction, a light detector is arranged on the side of the optical scanning start end of the light beam, and the timing from the detection of the light beam to the writing start is adjusted, so as to start the writing in the main-scanning direction.
Moreover, as to the sub-scanning direction, the position of the paper in which an image is formed is detected, and then the timing is adjusted, so as to start the writing.
In manufacturing, it is necessary to set the writing start position with respect to the timing assumed in the design-time by the adjusting the timing while considering the manufacturing errors of the every parts from the light source for generating the light beam to the scanned face (photoconductor) actually scanned by the light and the assembling errors of each part.
However, even if the timing is adjusted to an ideal state in manufacturing, the displacement of the writing start position is developed with the temporal change after the manufacturing. Moreover, the displacement of the writing start position is developed by the expansion and contraction of the parts caused by an environmental condition, especially, a temperature condition when using. These conditions have a significant impact especially on resin parts. Therefore, if the position of the light beam can be detected, the writing start position can be corrected by using an appropriate correction device. In addition, the method for correcting the writing start position by detecting the position of the light beam can be used for adjusting the timing in the manufacturing.
On the other hand, colorization of an image forming apparatus which conducts optical scanning corresponding to each of the colors (for example, cyan, magenda, yellow, and black) and overlaps each of the colors, so as to form a color image is recently improved. In such an image forming apparatus, if the displacement of the image writing start position of each color is developed, image deterioration, so-called color shift occurs in the overlapped color image. This is a problem.
In this case, if the position of the light beam corresponding to each color can be detected, the writing start position of each color is corrected by using an appropriate correction device, and a favorable color image can be obtained by aligning the writing start position of each of the colors.
A high-speed image forming apparatus is also improved. An optical scanning device for conducting optical scanning by means of a plurality of optical beams is increasingly used.
In this optical scanning device, a multi-beam light source having a plurality of luminous points (including an array light source mounted on one chip) is used as the light source instead of a conventional single beam light source having one luminous point.
In the optical scanning device using the multi-beam light source, image deterioration, so called uneven density is caused especially to the displacement in the sub-scanning direction by the change in the distance (pitches) of the light beams when scanning on the scanned face.
In this case, if the position of each of the light beams can be detected, a good image without having uneven density can be obtained by correcting the distance of the light beams by means of an appropriate correction device.
The technique for correcting the manufacturing errors of the parts or the technique for correcting the scanning line pitches of the multi-beams can contribute to the resource saving and the noise reduction in the high speed operation (speed reduction of a light deflector).
In order to achieve the above requirements, it is important to measure the position of the scanning light beams, and is desired to detect the position of the light beam especially in the sub-scanning direction and the distance of the light beams in the sub-scanning direction.
There is proposed in JP-H07-72388A, a technique (the first technique) for detecting a difference of distance of a plurality of light beams in the sub-scanning direction by means of a plurality of light beam detecting devices disposed in the main-scanning direction such that the end portion on the start side in the main-scanning direction of each of the light beam detecting areas becomes non-parallel each other. However, in this first technique, the plurality of detecting devices is arranged in the main-scanning direction, so the size in the light detectors increases in the main-scanning direction, resulting in the growth in the size of the optical scanning device.
The light detectors are generally arranged outside the effective image area in the main-scanning direction.
Accordingly, in addition to the effective image area width, it is necessary for the light beams of the scanning optical system to reach the light detectors. For this reason, if the size of the light detectors increases in the main-scanning direction, the size of the scanning optical system and also the size of the optical scanning device increase. The increase in the size of the scanning optical system causes increase in the length of the optical path and the high angle of field, and also the expansion in the effective diameter.
There is proposed in JP-H-09-325288A a technique (the second technique), which disposes a light shielding mask tilted at a predetermined angle in a sensor array having a plurality of photo-sensors arranged in the main-scanning direction, and calculates the pitches of the laser beams in the sub-scanning direction.
However, in this second technique, the sensor array in which the plurality of photo-sensors is incorporated has a particular shape. Also, the light shielding mask is required. Accordingly, the sensor array becomes expensive. In addition, photo-sensors are arranged in the main-scanning direction, so the size of the sensor array including a circuit board increases.
There is also proposes in JP-H10-235928A a technique (the third technique), which receives light entered from one side portion vertical to the scanning direction, disposes a plurality of light receiving elements for emitting from the other side portion tilted to the scanning direction, and calculates the difference of the scanning light in the sub-scanning direction. In this third technique, the light receiving elements are disposed in the sub-scanning direction; thereby, the increase in the size of the light receiving elements can be prevented, compared with the first technique and the second technique that the light receiving elements are disposed in the main-scanning direction. However, similar to the second technique, the sensor array in which a plurality of photo-sensors is incorporated has a specific shape, so the sensor array becomes expensive. In the third technique, the output signals from both of the incident side and the emission side (more particularly, both of the rising signal and the falling signal) are used relative to one light receiving element, but it is not desirable from detecting accuracy view.
More particularly, in a photo-diode, a rising time generally differs from a falling time. In a photo-diode which falls from High level to Low level when light enters (falling signal), the rising time is slower than the falling time at several times. Therefore, it is not desirable to use the rising signal from detecting accuracy view.
There is proposed in JP2005-37575A and JP2005-62597A a technique (the fourth technique), which has a plurality of light receiving faces for detecting laser beams, disposes a laser beam detector that at least one end adjacent to the light receiving faces is arranged at an angle, and detects the position of the laser beam in the sub-scanning direction. In this fourth technique, the detection accuracy is maintained by using the falling signal from the light receiving face vertical to the scanning direction and the falling signal from the light receiving face having an angle. However, similar to the second technique and the third technique, the plurality of light receiving faces integrally mounted has a specific shape, so it becomes expensive.
As described above, in the above technique, there is a problem that the size of the optical scanning device increases caused by a plurality of light detectors arranged in the main-scanning direction for detecting displacement of the light beam in the sub-scanning direction. Also, there is a problem that the size of the optical device increases and the optical scanning device becomes expensive because the light receiving sections have a complicated shape, and the light detector having a specific shape that a plurality of light receiving sections is integrally mounted (such as an expensive light detector and an increased light detector having a circuit board) is used.
For the forgoing reasons, there is a need for an optical scanning device and an optical scanning method for detecting a position of a light beam in the sub-scanning direction by using an inexpensive light detector without increasing the size of the device.